After 70 years on the throne, Thailand’s King Bhumibol Adulyadej has died.

The world’s longest-reigning monarch — who was born in Cambridge, Massachusetts, and has ruled the Southeast Asian country since 1946 — passed away on Thursday, the palace has announced. He was 88.

According to the palace, the king died “peacefully” at 3:53 pm local time at Bangkok’s Siriraj Hospital. “Even though the board of doctors has closely monitored and treated him to the best of its abilities, the king’s condition never improved but deteriorated until Thursday,” the palace said.

King Bhumibol had been ill for years, suffering from kidney failure. He last made a public appearance in January.

On Sunday, the royal palace released a statement saying he was receiving hemodialysis to filter his blood and was in unstable condition, breathing with the help of a ventilator, The New York Times reported.

The Times added that on Wednesday evening, the palace released a new health bulletin saying that he appeared to have a blood infection, which was causing low blood pressure and abnormal liver function.

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King Bhumibol was born in the U.S. while his father studied at Harvard. At the time, Bhumibol’s uncle, Prajadhipok, was king. But a revolution in 1932 stripped King Prajadhipok of most of his powers and he abdicated in 1935. The crown then went to Bhumibol’s older brother, Ananda Mahidol. But in 1946, he was found shot dead in his bedroom in Bangkok’s Grand Palace, which resulted in Bhumibol becoming king at 18 years old.

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The heir to the throne, Crown Prince Vajiralongkorn, who is the only son of King Bhumibol, is expected to take over. He and his three sisters arrived at Wednesday at his hospital bedside. The crown prince is a polarizing figure in Thailand, and lacks the popularity of his father.

Thailand remains under military rule following a coup in 2014. Even in ill health, the beloved King Bhumibol was considered a unifying and stabilizing figure in a country that’s been hit by political turmoil.